Fractionation with side stripping



Oct. 5, 1965 J. A. BYERLY ETAL 3,210,271

FRACTIONATION WITH SIDE STRIPPING Filed Feb. 19, 1962 LLI (9 (I I I O t() '04 MAIN FRACTIONATOR THEIR ATTORNEY United States Patent O 3,210,271 FRACTIQNATION WlTH SIDE STRIPPING James A. Byerly, East Oradell, NJ., and Egon L. Deering and Bron B. Geye, Houston, Tex., assignors to Shell Oil Company, New York, N.Y., a corporation of Delaware Filed Feb. 19, 1962, Ser. No. 174,078 6 Claims. '(Cl. 208-355) The invention relates to the fractional distillation of multi-component mixtures, such as petroleum fractions, to separate the same into more than two products, wherein the mixture is distilled in a main vertical distillation zone from which at least a top, a bottom and an intermediate stream are withdrawn from said zone, one of the latter two streams is separated in an auxiliary stripping zone into a residual and an overhead stream, and said residual stream is discharged as a product while at least part of the overhead stream is returned to the main distillation zone.

In conventional practice all of the said overhead product from the stripper is returned to the main distillation zone. ln most instances stripping is effected by adding steam to the stripping zone, and this steam is introduced into the main distillation zone together with the volatile components of the mixture being distilled.

The return of all parts of the stripper overhead to the main distillation zone, including steam when used, loads the main column with vapor, thereby limiting the capacity of the plant. Such total return was, however, heretofore believed to be necessary because the stripper overhead contains components which belong in more than one of the product streams from the main distillation zone. This could be avoided only by the economically unacceptable course of installing an additional distillation column for processing the stripper overhead.

This circumstance has heretofore restricted the depth to which stripping could be carried out, since taking too large an overhead product caused greater loading of the main distillation zone.

lt is an object of this invention to reduce the vapor load in the main distillation zone of operations of the type described above, whereby the capacity of the column can be increased, without the need to install an additional distillation unit.

A further object is to permit an increase in the depth of stripping which can be performed in the auxiliary stripping zone, with the production of an increased amountbf stripper overhead product, if desired by the use of 1ncreased amount of steam, Without thereby overloading the main distillation zone.

Still other objects are to provide an intermediate reflux in the main distillation zone; and to reduce the requirements of reflux of top product to the main distillation zone.

In summary, according to the invention the intermediate and/ or bottom product withdrawn from a main v ertical distillation zone is separated in an auxiliary stripping zone, usually with steam, into residual and vaporized products and the latter is severely cooled to .condense most of the liqueliable components of the original feed mixture; the only components of the vaporized products thus condensed are returned to the main distillation zone, at an intermediate level thereof. The uncondensed vapor from the stripping zone, usually containing steam, may be collected as a separate product or mixed with the top products from the main distillation zone.

To achieve a low pressure-drop system, a refluxcondensing stream is preferably circulated through the upper section of the stripper, which is equipped with condensing trays or sprays by which the said stream is introice duced to the upper part of the stripper to condense the said liqucfiable components in the stripper vapors. This circulating stream is cooled by any suitable means, such as an exchanger which uses the cold charge or any other coolant, either liquid or gas, on the cold side.

The residual vapors from the stripper, preferably relatively free of components of the original feed mixture, are condensed, either together with the top product from the main distillation zone or separately.

The process may be operated at any pressure, either subatmospheric, atmospheric or under pressure, in accordance with the characteristics of the mixture being distilled. When the vapor pressure differential between the stripping gas and the stripped vapor is great, carefully selected conditions for the reflux-condensing stream render it possible to return the condensed, stripped vapor free of condensed stripping gas to the main distillation zone.

The invention will be further described in connection with the accompanying drawing forming a part of this specification, the single view of which shows a flow diagram of a preferred embodiment,

It will be understood that the drawing shows a simplified scheme, for the purpose of emphasizing the inventive features, and omits features which are known, such as control devices, pressure and temperature recorders, feed pumps and heat exchange circuits.

Referring to the drawing, the distillation complex includes a main fractionating column 1 containing a plurality of contacting trays 2 to form a main, vertical distillation zone, an inlet 3 for a multi-component feed stream which is heated in a furnace 4 and, optionally, a bottom reboiler S for supplying heat to the bottom of the column when the inlet 3 is above the bottom tray and/ or when the heat from the furnace 4 is insullicient, said heat being supplied by a pipe 6 at a rate controlled by a Valve 7. The column has a plurality of outlets at different levels, e.g., three, namely, an outlet 8 at the top for a vaporous top product, a bottom outlet 9 for a liquid residual or bottom product, and an outlet 10 at an intermediate level for a side stream which may be vapor or liquid but is usually liquid. Liquid reflux may be provided in any suitable manner, eg., by a reflux line 11 containing a pump 12 by which condensate from the receiver 13 is returned. The receiver is a phase separator to which the overhead vapors are introduced after being cooled in a condenser 14, and from which oily components are discharged at 15, an aqueous phase is discharged at 16, and incondensibles rat 17. The pipe 17 may be Connected to a vacuum system, not shown.

18 and 19 are auxiliary distillation columns, commonly called side or residue strippers, which contain contacting means, such as trays 20, and define auxiliary stripping zones. The bottom product and the side stream are supplied to the auxiliary columns, preferably to the upper parts thereof as shown. A stripping medium such as steam is admitted to the bottoms of these columns by steam lines 21 and 22. Above the inlets for the streams from the main column are liquid draw-off trays 23 or 24 from which condensate is drawn off through pipes 25 or 26 and pumps 27 or 2S to cooling devices, such as heat exchangers 29 and 30. The cooled liquid from the latter is returned as reflux to the upper parts of the auxiliary columns via pipes 31 and 32 and suitable distributors, such as spray nozzles, not shown. ln the example shown, the heat exchangers are cooled by flowing all or a part of the fresh charge to be distilled, which is admitted to the process via a pipe 33, serially through the exchangers. The partially heated charge is then passed to the furnace 4 via pipe 34 for further heating. However, any cooling medium, which may in some instances be refrigerated, can

be used. A residual liquid stream, stripped of volatiles, is discharged from each auxiliary column at 35 or 36 and discharged as products via pumps 37 and 38. Uncondensed vapor is discharged at 39 or 40. These uncondensed vapor streams may be combined in a pipe 41 and passed through a condenser-cooler 42 and a valve 43 and then commingled with the overhead product from the main column in the receiver 13. However, we may pass the combined uncondensed vapor stream, instead, through a valve 44 to a separate receiver 45.

A part of t-he'condensate from the stripping column i8 is returned, prior to cooling, via a pipe 46 to the main column at a suitable level, such as 47, whereat the composition is reasonably well in equilibrium with that of the condensate, this level being usually above that of the inlet 3. Similary, condensate from the stripping column 19 is returned to the main column via a pipe 48 at a suitable level. Because the best level is not always known it is desirable to provide a plurality of inlets 49 and 5t), and to select the inlet point by a valve 51 or 52, one of which is normally open while the other is closed.

In operation, the condensate discharged from the stripping columns through pipes 25 and 26 is severely cooled in the cooling devices 29 and 30, to temperatures well sbelow their 'boiling points, and the cooled liquid is returned to these columns to condense most of the liqueiiable constituents of the initial stream. Only the condensed liquids are returned to the main column. Because the unc-ondensed vapors discharged at 39 and 40 are relatively free from components of the original mixture other than those which it is desired to include in the top product, there is no need t=o return these vapors to the -main column. The result is that the vapor loading in the main column is greatly reduced, leading to increased capacity. Moreover, because the greater volurne in vapor resulting from deeper stripping does not lead to increased vapor loading in the main column it is possible, with the invention, to strip the streams in the auxiliary columns to any desired degree without limitation by the capacity of the column. This adds flexibility t-o the operation of the system.

Additional .advantages are that, Iby returning to the main column cooled liquid instead of vapor as in prior practice, a degree of reiiux is provided. Further, because liquid instead of vapor is returned, it is possible to return the condensate to a somewhat lower level in the main column than was desirable in prior practice. Still further, the cooling effected by the 4condensate reduces the amount lof reflux which is required to be returned to the column by the pump 1 2; this reduces the load on the condenser 14.

Example A vacuum distillation column of the type shown at 1 in the drawing, was provided with conventional sideand residue-strippers, in which the stripper overhead was returned to the main column. It became desirable to increase the degree :of stripipn-g of the 27.3 API gravity feed (at 622 F.) to the vacuum side stripper to produce a middle .lubricating oil by increasing the flow of stripping stream by a `factor of 3.1, thereby increasing the removal of light ends from 27% to 32%. Oomputations indicated that this modification was not possible because the main fractionator could not take the additional vapor load.

By the use of the disclosed invention, such increase steam stripping not 1only became possible, 'but the internal flows in the section of the main fractionator above the point where the side stream is withdrawn were reduced 19% for internal reflux and 13% for vapor `flow, as determined by heat-balance computations. In this example one-half of the condensate discharged from the side stri-pper through the pipe 26 is returned to the main fractionator at a temperature of 500 F., and the other half is returned to the top via the pipe 32 at a temperature of 250 F. The uncondensed vapor, drawn off at 40, has a temperature of about 275 F.

We claim as our invention:

1. A method of fractionally distilling a multi-component hydrocarbon oil which comprises the steps of z (a) subjecting said oil to fractional distillation in a -main vertical distillation Zone,

(b) with-drawing top and `bottom streams lfrom different levels of :said zone and an intermediate stream from an intermediate level,

(c) transferring at least one of the two latter streams to an auxiliary stripping Zone,

(d) admitting `steam to the bottom of `said stripping zone and separating the transferred stream into at least a vapor portion and a residual liquid portion,

(e) withdrawing said residual liquid portion 'from the striping zone,

(f) cooling said vapor portion and thereby condensing the major part of the oil constituents of the vapor to form a condensate,

(g) withdrawing condensate formed in step (f) `from the striping zone separately from said residual portion, returning a part :of the withdrawn condensate to the main distillation zone, cooling another part thereof to a lower temperature and mixing the latter cooled part with the vapor portion in step (f) to effect cooling of the 4said vapor portion, and

(h) `withdrawing the uncondensed part of said vapor portion `from the stripping zone without return thereof to the main distillation zone.

2. A method of fractional distillation which comprises the steps of:

(a) subjecting a multi-component feed to fractional distillation in a main vertical distillation zone,

(b) withdrawing a top stream from an upper part and at least one other `stream from a lower part of said ldistillation zone,

(c) transferring. at yleast one stream withdrawn from said lower part of the distillation Zone t-o a stripping zone and therein separating the transferred stream into at least a vapor portion and a residual 4liquid portion by injecting a striping vapor into a lower part of the stripping zone,

(d) withdrawing said residual liquid portion from the stripping zone,

(e) cooling said vapor portion and thereby condensing a part of the vapor to form a condensate, and

(f) withdrawing a part of the condens-ate formed in (e) yfrom t-he stripping zone separately from said residual portion, returning a part of the withdrawn condensate to the main distillation zone, cooling another-part thereof to a lower temperature, and mixing the latter cooled part with the vapor portion in step (e) to effect cooling of the said vapor portion, and

(g) withdrawing the unc-ondensed part of said vapor portion from the stripping zone without return thereof to the `main distillation Zone.

3. Method as defined in claim 2 wherein the said uncondensed part of said vapor portion .from the stripping zone is combined with the top stream from the main distillation zone.

4. Method as defined in claim 3 wherein there are withdrawn from said main distillation Zone, yat different levels thereof, the said top stream, a 'bottom stream, and at least one additional intermediate liquid stream, and the last-mentioned stream is transferred to the stripping zone.

5. Method as defined in claim 3 wherein there are withdrawn from said main distillation zone, at different levels thereof, the sai-d top stream, a botto-m stream, and at least one additional intermediate stream, and the said bottom stream is transferred to the stripping zone.

6. Method as defined in claim 3 `wherein there are withdrawn from said main distillation zone, at different levels thereof, the .said top stream, a bottom stream, and at :least one additional intermediate liquid stream, and

both the last-mentioned streams are transferred to separate `2,5 34,87 0 12/ 50 Kraft 202-40 stripping zones and therein separated int-o vapor and 2,540,379 l2/51 Ridgway et al 208-354 residual portions, and steps (c), (d), (e), (f), and (Ig) 2,736,688 2/56 Kraft 208-355 are performed separately for each of said stripping zones. 2,911,452 11/59 Broughton 202-395 5 2,952,616 9/ 60 Hausch 208-354 References Cited by the Examiner UNITED STATES PATENTS FOREIGN PATENTS 2,029,528 2/36 Gard `2()3 355 X 386,075 1/ 33 Great BTaiH- l2,476,010 7/49 Wittlesey 202-40 X GEORGE D. MITCHELL, Examiner. 

1. A METHOD OF FRACTIONALLY DISTILLING A MULTI-COMPONENT HYDROCARBON OIL WHICH COPRISES THE STEPS OF: (A) SUBJECTING SAID OIL T FRACTIONAL DISTILLATION IN A MAIN VERTICAL DISTILLATION ZONE, (B) WITHDRAWING TOP AND BOTTOM STREAMS FROM DIFFERENT LEVELS OF SAID ZONE AND AN INTERMEDIATE STREAM FROM AN INTERMEDIATE LEVEL. (C) TRANSFERRING AT LEAST ONE OF THE TWO LATTER STREAMS TO AN AUXILIARY STRIPPING ZONE, (D) ADMITTING STEAM TO THE BOTTOM OF SAID STRIPPING ZONE AND SEPARATING THE TRANSFERRED STREAM INTO AT LEAST A VAPOR PORTION AND A RESIDUAL LIQUID PORTION, (E) WITHDRAWING SAID RESIDUAL LIQUID PORTION FROM THE STRIPING ZONE, (F) COOLING SAID VAPOR PORTION AND THEREBY CONDENSING THE MAJOR PART OF THE OIL CONSTITUENTS OF THE VAPOR TO FORM A CONDENSATE, (G) WITHDRAWING CONDENSATE FORMED IN STEP (F) FROM THE STRIPING ZONE SEPARATELY FROM SAID RESIDUAL PORTION, RETURNING A PART OF THE WITHDRAWN CONDENSATE TO THE MAIN DISTILLATION ZONE, COOLING ANOTHER PART THEREOF TO A LOWER TEMPERATURE AND MIXING THE LATTER COOLED PART WITH THE VAPOR PORTION IN STEP (F) TO EFFECT COOLING OF THE SAID VAPOR PORTION, AND (H) WITHDRAWING THE UNCONDENSED PART OF SAID VAPOR PORTION FROM THE STRIPING ZONE WITHOUT RETURN THEREOF TO THE MAIN DISTILLATION ZONE. 